Electrochemical energy storage technologies constitute important research fields for use in a myriad of applications ranging from aerospace and military, to consumer use such as plug-in automobile power source, stationary energy storage, and cellphone battery.
Lithium ion batteries (LIBs) form an important class of electrochemical energy storage devices. Lithium ion batteries have high energy densities of about 100 Wh kg−1 to about 200 Wh kg−1, with power densities of up to about 100 W kg−1. Energy storage mechanism of lithium ion batteries is associated with Faradic reaction between Li+ and bulk electrode materials. Despite the high energy density of lithium ion batteries, slow Li+ diffusion kinetics and poor reversibility of electrode reactions result in low power density and low cycling stability of lithium ion batteries of about 100 cycles.
Besides lithium ion batteries, supercapacitors belong to another class of important electrochemical energy storage devices, which provide transient but high power output for various machines and devices, such as trucks, buses, elevators as well as heavy duty construction machinery in powering forklifts and yard cranes, and in railways. Notably, supercapacitors have been used in electronics, transportation and energy applications with a market size of about 1.2 billion USD in 2015. Currently, there are about 51 major manufacturers in the world for supercapacitors.
Generally, supercapacitors possess high power delivery ability, high cycling stability of more than 100,000 cycles and high energy round trip efficiency of greater than 90%. Coupled with the almost non-existent geometric restriction or special requirement for supercapacitor cell installation, supercapacitors may provide great versatility and flexibility for energy storage markets.
Supercapacitors may generally be classified in the following two categories—electrical double layer capacitor (EDLC) and pseudocapacitor. Electrical double layer capacitor uses carbon-based materials, such as active carbon, templated carbon, carbon nanotubes, carbon aerogels, and graphene, as electrode materials, and energy storage is realized by reversible formation of electrical double layer at the electrode-electrolyte interface. High surface area carbon material is presently the dominant choice in industry. Electric double layer capacitors are already used in various applications, such as transportation, electronics, and energy. Maximum specific energy density of commercial electrical double layer capacitor device, however, may only reach a value in the range of about 5 Wh kg−1 to about 10 Wh kg−1 due to low capacitance of the carbon-based materials. Meanwhile, power density of electrical double layer capacitor may reach up to 10 kW kg−1 with long term stability of over 10,000 cycles.
Pseudocapacitor, on the other hand, usually uses different metal oxides such as such as MnOx, NiO, Co3O4, V2O5, or conducting polymers such as polyaniline, polypyrrole and related conjugated conducting polymer as electrode materials. Its energy storage mechanism is generally based on reversible faradic reaction in the near surface, which may be in the order of a few nanometers, of the material. As a result of the multi-electron electrochemical reaction, energy density of a pseudocapacitor may be much higher than that of an electrical double layer capacitor. Most studies are focused on pseudocapacitive material//carbon devices (so called asymmetric supercapacitor). The asymmetric supercapacitor may have an energy density of up to 40 Wh kg−1, while power density may be lower than that of an electrical double layer capacitor.
Due to the low energy densities of supercapacitors, a great number of supercapacitor modules are required to store energy for use in applications, such as energy storage of electricity in a power grid during off-peak hours. This has restricted their use in industry due to high costs and space needed to accommodate the supercapacitors. The energy density of supercapacitors is required to double or triple to a range from about 20 Wh kg−1 to about 30 Wh kg−1 at a certain power density for various emerging applications so as to penetrate a larger energy storage market.
Lithium ion capacitors (LICs) form a new generation of supercapacitors, involving use of a lithium ion battery electrode and an electrical double layer capacitor electrode placed in an organic electrolyte. The lithium ion battery electrode may be a positive electrode, such as LiCoO2, or LiMnO2, or a negative electrode, such as graphite, TiO2, or Li4Ti5O12. As compared to traditional supercapacitors, operation potential of lithium ion capacitors may be greatly enhanced due to larger electrochemical reaction potential difference between the negative electrode and the positive electrode in the organic electrolyte. Furthermore, high capacity of the lithium ion battery electrode may promote energy density of the overall lithium ion capacitor. Generally, a lithium ion capacitor may have an energy density in the range from 40 Wh kg−1 to 80 Wh kg−1, and a power density of about 1 kW kg−1. Cycling stability of the lithium ion capacitor may also be also superior to that of a lithium ion battery, and may reach up to a few thousand cycles.
Presently, electrical double layer capacitors, asymmetric supercapacitors, and lithium ion capacitors, form the three major technologies of electrical energy storage for large scale energy storage or as a small module for portable devices. As mentioned above, high surface area carbon-based material forms the essence in an electrical double layer capacitor device. As charge storage in electrical double layer capacitor involves charge adsorption/desorption in a surface of the carbon electrode, limitations exist in that carbon-based material has low capacitance. Though the carbon-based material is able to deliver high power density to the device, low capacity of the electrode in electrical double layer capacitor restricts overall device energy density. On the other hand, even though lithium ion batteries have high energy density, slow electrode kinetics limit power density of the device. Long term stability of the lithium ion batteries is also an issue. From the above discussion, it may be seen that limitations in state of the art technologies exist, and development of a device with both high energy density and high power density is of great value for broad applications.
In view of the above, there exists a need for an improved material that may be used in electrodes, as well as energy storage devices that overcome or at least alleviate one or more of the above-mentioned problems.